Apparatus for testing insulation



Dec. 23,1952 D. HANSEN E'IAL 2,623,100

APPARATUS FOR TESTING INSULATION Filed Oct. 12. 1949 5 Sheets-Sheet l FIG-l INVENTORS L.D. HANSEN D.E.OVERMYER ATi'ORNEV Dec. 23, 1952, 1.. D. HANSEN ETA]. 2,623,100

APPARATUS FOR TESTING INSULATION Filed Oct. 12, 1949 w 5 Sheets-Sheet z /vv/v TORS L .D. HANSEN D.E.OVERMVER By MM )1 i TORNEV Dec. 23, 1952 L. D. HANSEN ETAL 2,523,100

APPARATUS FOR TESTING INSULATION Filed Oct. 12. 1949 5 Sheets-Sheet 3 INVENTORS L.D. HANSEN 0. E .OVE RMVE R A TTOPNEV FIGS Dec. 23, 1952 1.. D. HANSEN ET AL 2,523,100

APPARATUS FOR TESTING INSULATION Filed Oct. 12, 1949 5 Sheets-Sheet 4 blm lllllllllllflllllli' 28 & immimix zwu 4 'IIIIAIMHIWIIIAIIIIIHIWIIIIIJ "HI I INVENTORS 34 as L. D. HANSEN D.E.OVERMYER .4 T' TORNEY Patented Dec. 23, 1952 UNITED STAT ES PAT EN T O F FlC-E APPARATUS FOR TESTING INSULATION LloydD. Hansen, Fanwood, and Dwight E'. Over myer, Plainfield; N; J., assignors-to Western Electric Company, Incorporated, New York, N. Y., .a corporation of New York Application October 12, 1949, Serial 'No. 120,912-

9-Glaimsi 1 This invention relates'tothe testing of materials andmore particularlytothe testingof insulating materials for the detection of cracks therein.

In the manufactureiof jacks and lamp mountings used in telephone switchboard's' and Sim"- ilar equipment, the mounting strips of dielectric material such as hard rubber which pro vide'the insulation betweenjacks or'lamp sockets are usuallygang drilled during the'manuia'cturing process to" receive the sleeves" of" the individual jacks, or' the sockets of" the lamps. Sinc'e the width of each strip'is madeas small as practicable in order' to permitth'e' maximum number of 'jacksor" l'amp' sockets to be installed in the switchboard; the'wall thickness of the bores is necessarily quite thin; Experience'h'as shown the-ta" certain'percen-tage of 'th'esemounting'strips' develop cracks in the drilling operation, and it is necessary that these cracks be detected and the parts discarded before they become assembled into complete mountings.

The detection of these cracks in insulating mountings" has heretofore been done visually. Since the insulating material frequently cracks in such a manner as to leave no visual indication, it has often been necessary to'fiex' the mountings'in the test inorder to exposeoracks of this type. 'Ihisprocess does not necessarily disclose all of'the' smaller incipient cracks, and in addition, flexingof themountings' in this type of testing results in the breaking of a certain percentage of satisfactory mountings in the flexing operation.

It is therefore an object of this invention to enable these insulating mountings to be quickly and accuratelyftested without thenecessity for mechanical stress.

Inone embodiment of the invention, applicant accomplishes this object by mounting a: high frequency oscillator on a motor driven carriage and providing the oscillator with. a. bifurcated output electrode which straddles a row of stationary electrodes on which the mountings "to" be tested are positioned. A test voltage applied only while the carriage is in motion produces a spark discharge through any cracks in the mounting and blocks a normally conducting vacuum tube, thereby actuatingan indicator.

Other objects and features will be apparent from the following detailed description taken in conjunction with the attached drawings in which:

Fig. 1 is a front elevational view of the cabinet containing the testing apparatus;

the line 3--3 of Fig; 2; showing the carriage driving mechanism;

Fig. 4 is a plan view of the stationary elec-- trode assembly on which theparts'to be tested are mounted;

Fig. 5 is a sectional view of the stationary electrode assembly, taken along the line Fe -'5' Fig. 6 is a sectional view showing how the" stationary electrode assembly is l'oc'ked'into-po sition, taken along the line 6-6of Fig. 5;

Fig. 7 is a schema-tic wiring diagram of the a plurality of metallic electrodes IaWhich'are' spaced and aligned=to correspond to the bores l5 of the mounting being tested; Themetallic electrodes l9 are mounted on a StripQ U, this strip being rigidly connected toa platform 21- througha spacer strip 2-! bymeans of" bolts 22 and spacers 23. The electrodes I'B' are positioned by both strip 25 and strip 2-7. The clear ance holes in strip 2 l are made sufficiently larger than adjacent surfaces of electrodes I9 to permit the electrodes 19 to align themselves with the corresponding bores l5 of the insulating mountings I4. The spacers 28 for electrodes l9 areslightly shorter than spacers Zeta-allow the electrodes IS the necessary horizontal position* ing so the electrodes i9 will align with holes IS. The platform 21- has a flange 24 which holds the platform in place-in a rectan'gu'lar' opening 25- of a plate 26', thus positioningthe' test fixture H3 in the'testset.

Two lugs 31 arefast'ened to and project below the strip 26 of thetestfixtu're l8; these liigs being used for lockingthe test fixtureinto po-' sition. The lugs aredesigned to fit int slots 32 of supports 33, and the testfixture is held in position byplunger's 34 having knobs 35,.the plungers passing through bores 36'of'supports 33 and through the aligned openings 31of the lugs 31. The test fixture'may 'be removed from, the test set by moving'the'knobs 35 to'the' left 3 against the pressure of springs 38, which action withdraws the plungers 34 from the openings 31 of the lugs 3|, allowing removal of the fixture.

The heads 4| of metallic electrodes I9 are designed to fit into the bores of the mounting I4 being tested, as shown in Fig. 5, with sufficient clearance being provided to take care of irregularities in the alignment of the mounting bores. These heads have reduced portions 42 immediately below the head to aid in the testing operation as hereafter explained.

Energy for the testing operation is supplied by an oscillator This oscillator may be of any type which is capable of generating frequencies of about 2.5 megacycles at voltages ranging from 10,000 to 30,000 volts. The oscillator, which is mounted on a carriage 52 is movable in either direction along a runway 53 by means of rollers 54 mounted on the carriage 52. The motive power for this movement of the oscillator is supplied by a reversible motor 55, the particular type used in the present embodiment being of the capacitor start-capacitor run, single phase, alternating current type. The output shaft of the motor 55 is connected through gears 51 and 58 to a driving pulley 50. Drive belting 60, which passes around a driven pulley BI and a takeup pulley 62, is firmly clamped to the oscillator carriage 52 by the bolt 53 and clamping strip 64. The oscillator is thus caused to move along the runway 53 when the motor is energized, with a switch 65 being used to reverse the direction of rotation of the motor and correspondingly the direction of travel of the oscillator.

The oscillator is provided with a bifurcated electrode H which is used to apply the oscillator output to the mounting undergoing test. This electrode projects through a longitudinal opening 12 in the cabinet II and is partially enclosed by a housing 13 having sides 14 of a transparent dielectric material such as methyl methacrylate. The front cover 15 should preferably be standard window glass to protect the operator from any ultraviolet rays resulting from the operation of the oscillator. The housing is rigidly connected to the main part of the oscillator through the opening 12 so that it moves with the oscillator during the travel of the latter along the runway 53. The electrode H is of light gauge flexible metallic material such as phosphor bronze, and is bufurcated to form prongs so as to allow the testing voltage to be applied simultaneously on both sides of the test mounting to insure the detection of cracks regardless of their location in the mounting. The oscillators ordinarily used for generating such high frequencies have very high impedance and hence very low power capacity. Even mounting strips free of defects impose some load on the oscillator due to the dielectric loss in the material. When a mounting strip has a crack on each side of a bore they are usually very nearly opposite the thinnest portion of the material. If the two furcations of the electrode pass both cracks simultaneously, the two low impedance paths in parallel may result in such a large reduction in available oscillator voltage that the detector does not operate. Staggering the furcations as in Fig. 1 makes more efficient use of the oscillator power and makes the detector more reliable in operation.

The electrical operation of the testing apparatus is shown on Fig, 7 of the drawings. With reference to the schematic wiring diagram shown in this figure, alternating current potential is 4 supplied from a source IOI, the supply line being provided with a disconnect switch I02.

Electrical power for the operation of the driving motor is provided by leads I03 and I04. Reversing switch is used for making and breaking the motor circuit, the switch comprising a. top movable contact I05 which makes with contacts I06 and I01, and a bottom movable contact I08 which make with contacts I00 and H0. Top movable contact I05 is connected to lead I03 as shown, while bottom movable contact I08 is connected to lead I04 through a lead III. Contacts I05 and I00, which move in unison as the switch 65 is moved, close contact II2 of microswitch II3 against contact II4 when the former are moved so as to make with contacts I06 and I09. Similarly, contacts I05 and I08 close contact II5 of microswitch IIB against contact II1 when the former two contacts are moved so as to make against contacts I01 and H0. This is accomplished by mechanically interlocking contacts II2 and. N5 of the two microswitches with contacts I05 and I08, this interlocking being arranged to insure that the contacts I05 and I08 make first and break last, thus preventing these contacts of switch 65 from interrupting the line currents of the motor circuit.

Limit switches I2I and I22 are operated by the movement of oscillator carriage 52. When the oscillator carriage is at its far left position, looking at Fig. 1, it will coact with limit switch I2I to break the contact WhlCh is normally made between contacts I23 and I24. When the oscillator carriage moves to the right away from its extreme left position, a spring return in the switch causes contacts I23 and I24 to remake. When the oscillator carriage reaches its extreme right position, it will coact with limit switch I22 to cause contact I25 to break with contact I26. This contact is similarly re-established when the carriage is disengaged from switch I22 after it has travelled a short distance to the left.

In energizing the driving motor 55, first assume that the carriage 52 is at its far left position, which means that contact I25 will be closed against contact I25. If it is desired to move the oscillator carriage to the right, switch 65 is moved to the right, which causes contacts I05 and I08 of the switch to make against contacts I06 and I09, and the movement of the switch 65 will also operate microswitch II3 to close contact II2 against contact II4. Power will now be supplied to one side of relay I3I through leads I03 and I32, contacts II2 and H4 of microswitch II3, contacts I25 and I26 of limit switch I22, and through lead I33 to the relay winding. The other side of relay I3I is connected to the power source through leads I54, I34 and I04. With the relay thus energized by the closing of switch 65, it will operate and cause relay contacts I36, I31, I38 and I39 to break with normally closed contacts I40, I4I, I42 and I43 and close against make contacts I44, I45, I46, and I41.

With the relay and its contacts thus operated, power is supplied to the four terminals A, B, C, and D of the driving motor as follows:

Terminal A is now connected to the main power source through lead I5I, contacts I39 and I41 of relay I3I, lead I52, contacts I09 and I08 of switch 05, and leads III and I04.

Terminal B is supplied with power through a path extending through lead I53, contacts I36 and I44 of relay I3I, and through leads I54, I34 and I04 to the source.

Terminal C is connected to the source along a path extending through lead I55, contacts I 38 and I46. of relay I3I, leads I56 and H31, contacts I96and I95 of switch 65, and lead I03.

Terminal D is connected to the power source through lead I58, capacitor I59, lead I60, contacts I31 and I95 of relay I3I, and leads Nil, I62, I63 and I93.

From the foregoing it will be seen that the driving motor is energized to drive the oscillator carriage 532 in a given direction when the contacts of relay I3I are closed by the movement of switch 65' to one of its two operating positions. The carriage may of course be stopped at any point along its travel by moving the switch G5; to its neutral position.

When the oscillator carriage has been moved to the right a distance sufficient for the completion of the test, the carriage 52'will coact with limit switch I22 to cause contact I25 to break from contact I25 and thus open the energizing circuit for relay I3I. When the relay releases, its contacts return to their normal positions, thus interrupting the power supply to the driving motor.

If it is desired to reverse the motor and move the oscillator carriage to the left, switch 65 is moved to the left, causing contacts I95 and I98 to make against contacts I91 and IE9, with the movement of the switch to the left also causing contact II5 of microswitch H8 to make against contact IN. This will cause relay ltI to become energized through a circuit extending on one side through leads I54, I34, and I04, and on the other side through leads I33, Ill and I72, contacts I24 and I23 of closed limit switch I2I, lead I13, contacts Ill and I I5 of mioroswitch I I5, and leads I63 and I93 to the power source.

When the movable contacts of relay it! close against the make contacts of the relay, motor terminals B and D are supplied with power through the same circuits as described previously for the opposite direction of rotation. Terminal A will now be supplied with power through a circuit extending through lead IEI, contacts I39 and I'll, lead I52, contacts I97 and I95 of switch 65, and lead I93. Terminal C will now be connected to the source through lead I55, contacts I38 and IE6, leads I56 and [13, contacts H9 and IE8 of switch 65, and leads III and I99.

When the power circuits for terminals A and C are thus reversed, the direction of rotation of the driving motor will be reversed, causing the oscillator carriage 52 to be moved to the left. After completion of the test in this direction, the carriage will be stopped as before by the coaction of the carriage with limit switch I2I.

As an aid in stopping the carriage quickly, provision is made for supplying direct current potential to the motor terminals as soon as the alternating current power circuit for the motor has been broken by the release of relay I3 i. The means for accomplishing this includes a rectifier unit I8I of a standard type, which is connected to the. main A. C. supply source on one side through leads I82, I92, I63, and I593, and on the other side through leads I93, ifi i, and I94. When the energizing circuit of relay IBI is broken, thereby causing the movable contacts of the relay to make against the normal contacts of the relay, direct current potential is supplied to terminal A of the driving motor through leads E84, i523, contacts M3 and I39, and lead I5I; to terminal B through lead I85, lead I96, contacts I49 and I36, and lead I53; to terminal C through lead I85, contacts I42 and I38, and lead I55; and to terminal D through lead I84. The application of direct current to the motor terminals ashereindescribed will stop the motor'by locking itsrotor; This dynamic braking in conjunction with the mechanical advantage of the gear train associated with the motor drive, will bring the motor to a quick stop after the actuation of the limit switches.

One of the main features of the invention is the use of the output of the oscillator 5| to control the grid circuit of vacuum tube 29 I and thereby indicate whether the mounting under test is satisfactory or unsatisfactory. The tube 20I includes a plate 292, a screen grid 203, a control grid 294, a suppressor grid 25I, and a cathode 205, the cathode and suppressor grid being grounded. The plate voltage is supplied from a suitable rectifier 296 which is connected to the 'A. C. source through leads 29'! and 298. the rectifier, one lead 299 is grounded while the other lead ZIIl is connected to the plate 202' through contacts ZII and 2I2 of relay 2I3, and through the winding of the relay 253 and lead 2M. The grid circuit of the tube 29I includes all of the metallic electrodes I9 in parallel, and, as shown in Fig. 7, the bifurcated electrode II is so positioned with respect to the electrodes I9 that the row of metallic electrodes I9 lies approximately equidistant from the two prongs I9 during the travel of the brush assembly.

The tube 29I is a normally conducting tube,

with the grid voltage normally being zero, which,

means that relay 2 I3 will be energized and cause contact 2H to be made against contact M2 and contact 22I to be separated from contact 222.

The energy which passes through the electrode ii is supplied from the oscillator 5|. The input side of the oscillator is connected to the power leads to? and 299 through a transformer 2 I5 and leads 2 It and 2Il'.

The test mounting which is positioned in the gap between the prongs it and the electrodes. I9 is subjected to high frequency oscillations at a voltage between 10,000 and 30,090 volts. These are damped wave trains of about 2.5 megacycles whose pattern is repeated times a second.

When a mounting I l which is free from defects.

is placed over the metallic electrodes I9, its dielectric strength is high enough to prevent. a spark from passing from the prongs it of the electrode II to any of the electrodes I9. The gap between the prongs '59 and the electrodes I9 acts in conjunction with the capacitor 2M to form in effect a capacity voltage dividing network, and when no breakdown is present the gap. presents a high impedance in comparison with the capacitor 99!, causing a large voltage drop acrossv the gap, and a small voltage drop across thecapacitor. The low alternatin voltage across the capacitor 24! is further attenuated by a low pass filter comprising a high frequency choke 242 and a resistor 2G3, and a grid leak circuit including capacitor 2M and resistor 245 prevents the attenuated voltage from blocking the normal conduction of the vacuum tube.

When a mounting I4 containing a crack or similar defect is tested, the dielectric strength at the point of the defect will be reduced to a point which allows a spark to jump from nearest prong I6 oi the electrode 'II to the metallic electrode I9 nearest the defect when the electrode II passes the defect during the travel of the oscillator carriage. The oscillator output may be varied by potentiometer 225 to set the voltage gradient between the electrode II and the metallic electrodes I9 at a value high enough to insure the passing On the D. C. side of" of a spark between the two when a crack is present in the mounting being tested.

When a spark passes across the gap through the crack in the mounting, the gap presents a low impedance path in comparison with the capacitor 24!, which causes a high alternating voltage to appear across the capacitor. On the first succeeding positive half cycle, the control grid 204 of the tube is made momentarily positive with respect to the cathode 205 by virtue of the voltage now impressed across the grid circuit, and the resultant grid current flow charges capacitor 244. The charged capacitor places a negative voltage bias on the grid of the vacuum tube 2M which is sufficient to cut oif or reduce the average current in the plate circuit of the vacuum tube and to a point which allows relay 2 l3 to release. The time constant of the circuit of capacitor 244 and resistance 245 is chosen to retain this negative bias on the grid for a period long enough to allow relay 2 l3 to become deenergized.

The release of relay 2l3 causes contact 2 to break from contact 2|2, which breaks the plate voltage circuit. This breaking of the plate voltage circuit means that the relay 2l3 will not reclose when the spark surge ceases and insures a positive indication for a short interval surge. At the same time, the release of relay 2 l3 causes contact 22 I, which is mechanically interlocked with contact 2, to make against contact 222, and thereby close an indicating circuit which is energized through leads 226 and 227. A lamp 228, which is connected in the indicating circuit, is energized by the closing of contacts 22l and 222 to indicate the fact that the mounting under test is defective. A buzzer 229 is connected in the same circuit through a switch 230 to provide optional audible indicating means to supplement the lamp 228.

The screen grid 203, which is connected to the plate circuit through lead 23| and a variable resistor 232, is used to adjust the operation of the plate circuit to insure that the tube will conduct when a satisfactory mounting is being tested, and

that the current in the plate circuit will be cut off or reduced sufficiently when a defective mounting causes a spark to pass between the electrodes H and I9.

After a defective mounting has been removed from the test fixture, the tube 20! is returned to its normally conducting condition by pressing a momentary contact reset button 235. This reestablishes the normal plate potential through leads 210, 23!, 236, reset button 235, the winding of relay 2|3, and lead 2M. Closure of this circuit energizes relay 2l3 which locks up through contacts 2| I and 212.

It is desirable to place a shield 252 in the control grid circuit to render it insensitive to any influence other than the oscillator spark and to insure that the spark is not diverted from its intended path.

As a safety feature, the oscillator power supply circuit is interlocked with the motor drive circuit to cause the supply to be disconnected when the oscillator carriage is stationary at either end of its travel. This prevents the oscillator output from causing excessive dielectric heating or being otherwise a possible source of danger during such times as the oscillator is not in actual use. This is accomplished by connecting the transformer lead 2i! to the main supply lead 201 through the limit switches |2| and I22 and the microswitches H3 and H6. When the oscillator carriage is in its far left position, for example,

8 and it is desired to move it to the right, the transformer is energized through the limit switch I22, which is closed when the carriage is in this position, but the energizing circuit is not completed until contact N2 of microswitch H3 is closed against contact H4 by the moving of the switch 65 to its right position. Similarly, in moving the carriage to the left from its extreme right position, the energizing circuit for the transfomer 215 passes through closed limit switch l2l, but this circuit is not completed until the microswitch H6 is closed by the movement of switch 65 to the left.

It is, of course, possible to move the oscillator carriage 52 back and forth manually, thus eliminating the need for using a motor drive forthis purpose. However, where the carriage is moved manually, there is danger that the oscillator voltage may cause undue dielectric heating if the carriage remains for some time in one position, which may result in the burning of a good mounting. It is therefore recommended that some type of automatic drive be used to insure a definite timed movement of the oscillator carriage from one extreme position to the other. Mounting the oscillator on the carriage with its electrode permits the use of a short lead between the oscillator and its electrode. In addition, making the test fixture stationary and moving the oscillator relative to it enables the test mountings to be loaded and unloaded in the same position.

The heads 4| of electrodes I9 are provided with reduced portions 42 immediately below the head to prevent a spark from passing directly between the electrode H and the electrodes [9 and bypassing the mounting under test, which might happen when a mounting free from defects has been improperly placed on the testing fixture so as to leave an air gap between the bottom of the mounting and the top of the testing fixture. By reducing the bottom part of the electrodes l9 as shown, a longer air gap is provided between the two types of electrodes, thus minimizing the chance of a spark jumping across under these conditions.

It is essential that the platform 2| be of a dielectric material which will not carbonize when subjected to heating from the oscillator power and thereby provide an alternative short circuiting path through which a spark or current could pass between the electrode H and the metallic electrodes I9.

It is to be understood that the above described arrangements are simply illustrative of the application of the principles of the invention. Numerous other arrangements may be readily devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

What is claimed is:

1. Apparatus for detecting cracks in insulating material specimens having a plurality of aligned bores, comprising a first electrode, a group of secondary electrodes having the same spacing and alignment as the bores and being shaped to enter the bores, a source of electrical energy, an output circuit for the source including all of the electrodes and the specimen, means for moving the electrode units relative to each other, means for adjusting the magnitude of the source output to a value suflicient to break down the dielectric resistance of a specimen containing cracks, but insuflicient to break down the dielectric resistance 01' a specimen free from cracks,

an indicating device, and means responsive to a breakdown condition of the dielectric strength of the specimen for actuating the indicating device.

2. Apparatus for testing insulating mountings having a plurality of aligned bores comprising a plurality of aligned stationary electrodes having the same spacing and alignment as the bores and being shaped to enter the bores, a carriage, an electrode mounted on the carriage, a motor for traversing the carriage between two limiting positions to move the carriage electrode along the aligned electrodes, a normally inactive oscillator connected to the electrodes, means for energizing the motor to drive the carriage se-- lectively in either direction, a circuit for supplying power to energize the oscillator and a switch actuated in each limiting position of the carriage to stop the motor and deenergize the oscillator.

3. Apparatus for testing the dielectric strength of insulating mountings having at least one bore comprising a test fixture having at least one stationary electrode for receiving a mounting to be tested, said electrode being shaped to fit the bore, a high frequency oscillator having a discharge electrode, an energizing circuit for the oscillator, a carriage for moving the oscillator and its electrode through an arcing discharge position with respect to the stationary electrode, a motor for driving the carriage, a circuit for the motor, limit switches included in the circuits and operated by the carriage for stopping the motor at the extreme carriage positions and de-energizing the oscillator, and a circuit controlled by the limit switches for electrically braking the carriage.

4. Apparatus for testing insulating mountings comprising a cabinet having a frontal shelf and an elongate opening above the shelf, a test fixture mounted on the shelf having a plurality of aligned electrodes extending upwardly through the shelf to receive mountings to be tested, a carriage within the cabinet, a test oscillator mounted on the carriage and connected to the electrodes, means for traversing the carriage between limiting positions, an output lead for the oscillator extending through the elongate opening and terminating in an output electrode which moves parallel to and in spaced relation with the aligned electrodes, an electrical. circuit including the output electrode and the aligned electrodes, and means connected in the circuit to indicate the passage of current between the electrodes, a start switch for connecting the oscillator and carriage traversing means to a source of power, and a switch actuated in each limiting position of the carriage to stop motor and deenergize the oscillator.

5. Apparatus for detecting cracks insulating mountings, comprising a cabinet, a test fixture mounted on the cabinet and having a plurality of aligned electrodes, an oscillator having furcated output electrode, an output circuit for the oscillator including the fixture electrodes and the mounting to be tested, with the mounting being physically and electrically positioned between the fixture electrodes and the furcations, a carriage supporting the oscillator, driving means for the carriage for causing the oscillator to traverse the test fixture in the line of the fixture electrodes, means for energizing the oscillator to apply electrical stress across the test mounting, means for adjusting the magnitude of the osci1 lator output to a value sumcient to cause a current surge between the furcated electrode and a fixture electrode when the test mounting contains a crack, but insufficient to break down the dielectric resistance of the test mounting when the mounting is satisfactory, a normally conducting vacuum tube including a cathode and a control grid, with the grid connected in the oscillator output circuit and attaining a voltage sufiiciently negative with respect to the cathode to block the operation of the tube on the occurrence of said current surge, and means re- 'sponsive to a blocked condition of the tube for indicating a defective mounting.

6. Apparatus for detecting cracks in insulating mountings, comprising a cabinet, a test fixture mounted on the cabinet and having a plurality of'ali'gned electrodes, an oscillator having a bifurcated output electrode, an output circuit for the oscillator including the fixture electrodes and the mounting to be tested, with the mounting physically and electrically positioned between the fixture electrodes and the furcations, a carriage supporting the oscillator, driving means for the carriage for causing the oscillator to traverse the test fixture in the line of the fixture electrodes, means for energizing the oscillator to apply electrical stress across the test mounting, means for adjusting the magnitude of the oscillator output to a value sufiicient to cause a current surge between the bifurcated electrode and a fixture electrode when the test mounting contains a crack, but insufiicient to break down the dielectric resistance of the test mounting when the mounting is satisfactory, means for interlocking the oscillator energizing means with the driving means to disconnect the energizing means when the carriage is stationary, a normally conducting vacuumtube including a cathode and a control grid, with the grid being connected in the oscillator output circuit and attaining a voltage sufiiciently negative with respect to the cathode to block the operation of the tube on the occurrence of said current surge, and means responsive to a blocked condition of the tube for indicating a defective mounting.

'7. Apparatus for detecting cracks in insulating mountings, comprising a cabinet, a test fixture mounted on the cabinet and having a plurality of aligned electrodes, an oscillator having a furcated output electrode with the furcations staggered with respect to the aligned electrodes, an output circuit for the oscillator including the fixture electrodes and the mounting to be tested, with the mounting being physically and electrically positioned between the fixture electrodes and the furcations, a carriage supporting the oscillator, driving means for the carriage for causing the oscillator to traverse the test fixture in the line of the fixture electrodes, said means being dynamically braked when the oscillator has completed its traverse, means for energizing the oscillator to apply electrical stress across the test mounting, means for adjusting the magnitude of the oscillator output to a value sufiicient to cause a current surge between the bifurcated electrode and a fixture electrode when the test mounting contains a crack, but insufiicient to break down the dielectric resistance of the test mounting when the mounting is satisfactory, means for interlocking the oscillator energizing means with the driving means to disconnect the energizing means when the carriage is stationary, a normally conducting vacuum tube including a cathode and a control grid, with the grid con- 1 1 nected in the oscillator output circuit and attaining a voltage sufllciently negative with respect to the cathode to block the operation of the tube on -the occurrence of said current surge, and means responsive to a blocked condition of the tube for indicating a defective mounting.

8. Apparatus for detecting cracks in insulating mountings, comprising a cabinet, a test fixture mounted on the cabinet and having a plurality of aligned electrodes, an oscillator having a bifurcated output electrode, an output circuit for the oscillator including the fixture electrodes and the mounting to be tested, with the mounting being physically and electrically positioned between the fixture electrodes and the furcations, a carriage supporting the oscillator, driving means for the carriage for causing the oscillator to traverse the test fixture in the line of the fixture electrodes, means for energizing the oscillator to apply electrical stress across the test mounting, means for adjusting the magnitude of the oscillator output to a value sufiicient to cause a current surge between the bifurcated electrode and a fixture electrode when the test mounting contains a crack, but insufficient to break down the dielectric resistance of the test mounting when the mounting is satisfactory, a normally conducting vacuum tube including a cathode and a control grid, with the grid connected in the oscillator output circuit and attaining a voltage sufliciently negative with respect to the cathode to block the operation of the tube on the occurrence of said current surge, means responsive to a blocked condition of the tube for indicating a defective mounting, and means for restoring the tube to its normal condi tion after the cessation of the current surge.

9. Apparatus for detecting cracks in insulating mountings, comprising a cabinet, a test fixture mounted on the cabinet and having a plurality of aligned electrodes, an oscillator having an output electrode both bifurcated and staggered, an output circuit for the oscillator including the 12 fixture electrodes and the mounting to be tested, with the mounting being physically and electrically positioned between the fixture electrodes and the furcations, a carriage supporting the oscillator, driving means for the carriage for causing the oscillator to traverse the test fixture in the line of the fixture electrodes, said means being electrically braked when the oscillator has completed its traverse, means for energizing the oscillator to apply electrical stress across the test mounting, means for adjusting the magnitude of the oscillator output to a value suflicient to cause a current surge between the bifurcated electrode and a fixture electrode when the test mounting contains a crack, but insufficient to break down the dielectric resistance of the test mounting when the mounting is satisfactory, means for interlocking the oscillator energizing means with the driving means to disconnect the energizing mcans when the carriage is stationary, a normally conducting vacuum tube including a cathode and a control grid, with the grid connected in the oscillator output circuit and attaining a voltage sufficiently negative with respect to the cathode to block the operation of the tube on the occurrence of said current surge, means responsive to a blocked condition of the tube for indicating a defective mounting, and means for restoring the tube to its normal condition after the cessation of the current surge.

LLOYD D. HANSEN. DWIGHT E. OVERMYER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,195,504 Stone Apr. 2, 1940 2,280,119 Gorman et al Apr. 21, 1942 2,304,710 Schmidt Dec. 8, 1942 2,436,615 Stearns Feb. 24, 1948 

